The present invention relates to an electronic circuit device and a manufacturing method therefor and more particularly to an electronic circuit device which is mounted in an engine room or a transmission of a car and controls an engine and an automatic speed regulator.
The method for controlling an automatic speed regulator of a car by an electronic circuit is widely adopted and there are various mounting structures of such an electronic circuit available. An electronic circuit device with a transmission directly attached is selectively installed in a location free of damage to the electronic circuit due to entering of water and oil and is structured so as to prevent them from entering.
As an example of the structure, an electronic circuit substrate is mounted on a metallic base, and the base and a cover are joined by resistance welding or laser welding, and the electronic circuit device is air-tightly sealed and charged with inert gas such as nitrogen. This is known as a hermetic seal structure.
In this structure, the input-output terminal of the electronic circuit is pulled out from a through-hole formed in the base and the through-hole is sealed by glass having a high insulation resistance. Therefore, it is necessary to select optimal linear expansion coefficients of the base, glass, and terminal.
Namely, the reason is that when glass is fused at a high temperature such as several hundreds° C. to 1000° C. and returned to normal temperature, between the aforementioned three parts, residual compressive stress must be mutually acted.
The materials of these parts are limited and for example, when soda barium glass is used as a sealing medium, the base material is a steel plate and the terminal is composed of a combination of iron and nickel alloy (50% of iron and 50% of nickel). Therefore, the following problems are imposed.
(1) Oxidation prevention at high temperature is necessary and the base requires nickel plating having a high fusion temperature.
(2) For welding, the cover combination material is limited and the same steel plate as the base is used. When many heating elements are used in the electronic circuit, to facilitate heat dissipation, aluminum or copper is suitable for the base material. However, as mentioned above, a steel plate must be used and the heat dissipation of the steel plate is bad.
(3) In resistance welding, in order to make the electric contact resistance between the base and the cover uniform, the flatness accuracy of the two must be increased, so that there are some factors of increasing the cost.
(4) In laser welding, the welding is adversely affected by variations in the nickel plating thickness of the base and the bead of the welded part is exposed. Protective coating for preventing corrosion generation due to it is required.
(5) It is difficult to surely decide acceptance or rejection of welding from appearance and to confirm the air-tightness, the checking method using helium gas and the bubble leak checking method for checking for generation of air bubbles in an inert fluid are required.
To improve the aforementioned respects, a packaging art for adhering a circuit substrate with electronic circuit elements loaded to a base manufactured with a heat conductive material and by exposing one side of the base, sealing the circuit substrate and base with epoxy resin (mold resin) is proposed.
However, in the structure that silicone chips are used as electronic circuit elements, and a glass-ceramic substrate having a linear expansion coefficient close to that of the silicone chips is used as a circuit substrate, and these members are sealed with epoxy resin, there is a problem imposed as indicated below.
Namely, in the epoxy resin mold process (transfer mold process) of embedding the circuit substrate and base, during cooling of the mold resin (epoxy resin) after taking out it from the mold after the curing process, due to cure shrinkage of the epoxy resin and the difference in linear expansion coefficient from the substrate and base, on the boundary surfaces between the epoxy resin and the circuit substrate and base, separation or resin cracking is easily generated.
The equivalent linear expansion coefficient of epoxy resin during molding is larger than that of the base and the linear expansion coefficient of the circuit substrate is smaller than that of the base. Therefore, the circuit substrate and base are warped due to the shrinkage stress of the epoxy resin closely adhered to the circuit substrate, and the part of the epoxy resin not closely adhered or the part of weak adhesive force is applied with tensile stress, and the boundary surface is separated.
Further, the equivalent linear expansion coefficient of epoxy resin is composition of chemical shrinkage of the mold resin when it is cured in the mold, the linear expansion coefficient α2 between the molding temperature and the glass transition point Tg in the course of cooling to the room temperature after taking out from the mold, and the linear expansion coefficient α1 between the glass transition point Tg and the room temperature and it is a value about 4 times of the linear expansion coefficient at normal temperature.
To eliminate this separation, for example, when an adhesive film forming process is performed on the end face of the base, a problem arises that epoxy resin in the neighborhood of the boundary surface is cracked.
As a countermeasure therefor, use of epoxy resin having little cure shrinkage and a small linear expansion coefficient or performing of a flexible resin coating process of absorbing the difference in linear expansion coefficient on the base end face may be considered. However, in both cases, an increase in cost is unavoidable, thus an inexpensive structure is desired.
Further, in the prior arts, for example, there are a method, as described in Japanese Application Patent Laid-open Publication No. Hei 07-240493, for loading semiconductor elements on a lead frame and embedding and packaging these parts in mold resin and a method, as described in Japanese Laid-open Patent Publication No. Hei 01-205556, for packaging an IC chip, mounting members thereof, and a heat radiation board. These prior arts are not intended to package not only the electronic circuit substrate but also the base thereof in mold resin.
Further, as described in Japanese Application Patent Laid-open Publication No. Hei 06-61372 and Japanese Laid-open Patent Publication No. Hei 09-307026, a method for loading semiconductor elements on a substrate and sealing them with resin is available. However, for application of it to an art for sealing and mounting not only an electronic circuit substrate having a far larger area but also a base with one mold resin, there is a very big problem caused.